In interferometer applications it is necessary to move one or more surfaces extremely smoothly and accurately to within a fraction of a wavelength. Typically, in interferometry the wavelengths are quite small, for example 6,000 .ANG.. This is equivalent to 0.6 .mu.m, or less than 0.000001 meter. Usually in an interferometer one of the mirrors is moved relative to the other by a carriage on tracks. There are thus two sources of error in mirror position. One is the tracks, the other the wheels, ball bearings, or other bearings which ride on the tracks. As a result of the typical mechanical tolerances of the track and bearings, there may occur sheer error, in which the mirror moves slightly in its own plane. Angular error may also occur, in which the mirror rotates about an axis in its own plane. Velocity error may also occur when there is non-uniform motion of the carriage and mirror along the tracks. Any deviation from flatness or smoothness in the tracks, or roundness or smoothness of the wheels or bearings, can cause one or more of these errors as the carriage moves along the tracks.
In order to avoid such errors, a step scan approach has been used in which the mirrors are moved in discrete steps, and measurements are taken only after the carriage has come to a complete stop. This, of course, eliminates velocity errors, as there is no movement when the data is taken. However, while such a technique is useful in laboratory environments, it is not practical in some other uses because it requires more time to start, step, stop and take data. It also still suffers from some sheer and angular errors, although these could be corrected after the carriage has stopped after each step.
Linear motor drives are used to drive carriages on ball bearings on tracks. While the linear motor drive does reduce velocity error, the tracks or ways require extremely close mechanical tolerances in order to eliminate other errors. In addition, linear motors, e.g. speaker coils, produce only relatively limited travel.
Another technique utilizes a flex-pivoted parallelogram structure in which flex-pivots made of tempered bands of metal twist up to approximately 15.degree. without friction. This approach reduces velocity error but it introduces sheer error because the mirror must move vertically as well as laterally with the parallelogram action. In addition, such devices have limited travel and require fine adjustment of pivots to avoid angular error.